Method of surface-treating aluminum material for dissipating heat

ABSTRACT

The present application relates to a method of surface-treating an aluminum material for dissipating heat, which is capable of increasing the radiation heat flux to thus enhance heat dissipation performance, and includes anodizing an aluminum material using an electrolyte composed of oxalic acid, and forming cobalt sulfide (CoS) in surface pores of the aluminum material thus sealing the surface of the aluminum material.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority of Korean Patent Application Number 10-2014-0073159 filed on Jun. 16, 2014, the entire contents of which application are incorporated herein for all purposes by this reference.

TECHNICAL FIELD

The present application relates to a method of surface-treating an aluminum material for dissipating heat, and more particularly, to a method of surface-treating an aluminum material for dissipating heat, which is capable of increasing the radiation heat flux to thus enhance heat dissipation performance.

BACKGROUND

An aluminum material is light and has high thermal conductivity and electrical conductivity. Further, when an aluminum material is subjected to surface treatment, it may be enhanced in corrosion resistance and mechanical performance and is thus widely utilized in various fields. In particular, an aluminum material has been mainly utilized in vehicle parts because of its properties.

Use of an aluminum material for vehicle parts f is primarily intended to achieve lightness and to enhance heat dissipation performance. Conventionally, aluminum has been selectively used for parts requiring lightness or heat dissipation performance.

However, with an increase in vehicle technology, performance required of parts for vehicles has increased, and thus thorough research into ensuring lightness and high heat dissipation performance of an aluminum material is ongoing.

To achieve lightness, techniques for improving properties of an aluminum material by adjusting the composition of an aluminum alloy have been variously proposed.

However, it is difficult to enhance heat dissipation performance by adjusting the composition of an aluminum alloy. Accordingly, there is a need for a technique for increasing heat dissipation performance of an aluminum material through surface treatment.

Typically useful is an aluminum material having enhanced corrosion resistance and wear resistance through surface treatment such as anodization. Also, a film resulting from anodization has many pores and thus may exhibit a variety of colors through coloring using a dye or the surface thereof may be sealed through impregnation with a functional material.

Anodization for surface treatment of an aluminum material is generally performed by virtue of a sulfuric acid process using as an electrolyte a 10˜18 wt % sulfuric acid aqueous solution. The reason why such a sulfuric acid process is employed is that the electrolyte is the cheapest and power consumption is low, thus generating economic benefits. The anodization technique using a sulfuric acid process is aimed to enhance wear resistance and corrosion resistance of an aluminum material, but does not take into consideration heat dissipation performance of the aluminum material.

Recently, there are devised techniques for performing anodization treatment in a manner that enhances performance and properties of the aluminum material with the use of electrolytes having various compositions.

For example, when an electrolyte composed mainly of citric acid instead of sulfuric acid is added with oxalic acid, the resulting oxide film layer may have a porous structure which is formed regularly and stably, which is disclosed in “Method of forming anodizing electrolyte of aluminum alloy material and composition therefor” (Patent Document 1).

In Patent Document 1, as the porous structure of the oxide film is regularly and stably formed, the electrolyte may be prevented from remaining to thereby obviate the post treatment process. Furthermore, the aluminum material is enhanced in terms of not only chemical and mechanical properties including surface strength, corrosion resistance, wear resistance, insulating properties and heat resistance, but also electrical properties including voltage resistance. However, no consideration is given of heat dissipation performance of the aluminum material.

Typical examples of sealing treatment for finishing the surface of the anodized aluminum material include a boiling water sealing process, a low-temperature sealing process (NiF₂), etc. This sealing treatment process takes account of only the protection of the oxide film on the aluminum material, without the consideration of heat dissipation performance of the aluminum material.

SUMMARY

Accordingly, the present application has been developed keeping in mind the above problems encountered in the existing art. An object of the present application is to provide a method of surface-treating an aluminum material for dissipating heat, which may enhance heat dissipation performance using a surface treatment process and a sealing treatment process of an aluminum material.

In order to accomplish the above objective, an embodiment of the present application provides a method of surface-treating an aluminum material for dissipating heat. The method includes anodizing an aluminum material with an electrolyte comprising oxalic acid. The surface of the aluminum material is sealed by formation of cobalt sulfide (CoS) in surface pores of the aluminum material.

The oxalic acid of the electrolyte upon anodizing may have a concentration of 0.2˜0.8 M.

The electrolyte upon anodizing may have a temperature of 10˜40° C.

The anodizing may be performed for at least 30 min.

The sealing may primarily include immersing the anodized aluminum material in a cobalt acetate solution; and secondarily immersing the aluminum material in an ammonium sulfide solution.

Upon primary immersion, cobalt acetate (Co(CH₃COO)₂) of the cobalt acetate solution may have a concentration of 100˜250 g/L.

Upon secondary immersion, ammonium sulfide ((NH₄)₂S) of the ammonium sulfide solution may have a concentration of 10˜50 g/L.

According to embodiments of the present application, an electrolyte for use in anodization includes only oxalic acid, so that the color of the resulting oxide film is closer to black, thus enhancing the heat dissipation performance of the aluminum material.

Also, according to the present application, CoS is formed in the pores of the anodized surface, so that the surface color of the aluminum material is much closer to black, thus enhancing the heat dissipation performance of the aluminum material.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present application will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1A illustrates the surfaces of oxide films depending on the temperature and time period when conventional anodization using a sulfuric acid electrolyte was performed;

FIG. 1B illustrates the surfaces of oxide films depending on the temperature and time period when anodization using an oxalic acid electrolyte according to the present invention was performed;

FIG. 2A illustrates the relative radiation results of the oxide films depending on the temperature and time period when conventional anodization using a sulfuric acid electrolyte was performed;

FIG. 2B illustrates the relative radiation results of the oxide films depending on the temperature and time period when anodization using an oxalic acid electrolyte according to the present application was performed;

FIG. 3 illustrates the surface photographs and the relative radiation results of the aluminum materials after conventional sealing treatment and the sealing treatment according to the present application;

FIG. 4A illustrates changes in the radiation heat flux depending on the concentration of cobalt acetate in the course of primary immersion for sealing treatment using a black sealing process according to the present application; and

FIG. 4B illustrates changes in the radiation heat flux depending on the concentration of ammonium sulfide in the course of secondary immersion for sealing treatment using a black sealing process according to the present application.

DETAILED DESCRIPTION

Hereinafter, a detailed description will be given of embodiments of the present application with reference to the appended drawings. The present application is not limited to the following embodiments and may be variously modified, and the present embodiments are merely intended to complete the disclosure of the present application and are apparent to those having ordinary knowledge in the art within the scope of the present application.

It is typically known in the art that an aluminum material increases in radiation heat flux as the color of an oxide film formed thereon by surface treatment is closer to black, thus enhancing heat dissipation performance.

Accordingly, the present application addresses a method of surface-treating an aluminum material for dissipating heat, wherein conditions for anodization and sealing treatment that are applied to an aluminum material are improved, so that the surface color of the aluminum material is closer to black.

Particularly, the method of surface-treating the aluminum material for dissipating heat according to the present application includes anodizing an aluminum material with an electrolyte comprising oxalic acid, and sealing the surface of the aluminum material by formation of cobalt sulfide (CoS) in surface pores of the aluminum material.

In the method according to the present application, anodizing is a step of subjecting the surface of the aluminum material to anodization to form an oxide film closer to black thereon. The electrolyte used for anodization may contain only oxalic acid.

As such, the concentration of oxalic acid is set to 0.2˜0.8 M, and preferably 0.3 M. Since the saturated concentration of oxalic acid at 0° C. is 0.3 M, when the concentration of oxalic acid is less than 0.2 M considering the electrolyte temperature, the power necessary for performing anodization may increase, and the density of surface pores of the oxide film may decrease. In contrast, when the concentration of oxalic acid is higher than 0.8 M, oxalic acid is not further dissolved. Hence, the concentration of oxalic acid is preferably set to 0.2˜0.8 M.

The electrolyte used for the present embodiment preferably contains only oxalic acid. Alternatively, the electrolyte may further include an acid typically useful for anodization while mainly containing oxalic acid. For example, the electrolyte may include sulfuric acid, phosphoric acid or chromic acid, in addition to oxalic acid. As such, the concentration of sulfuric acid, phosphoric acid or chromic acid is preferably set to 0.1˜1 M.

When the concentration of oxalic acid in the electrolyte falls in the range of 0.2˜0.8 M, a current of 1˜5 ASD and a voltage of 50˜150 V may be employed upon anodization.

The temperature of the electrolyte upon anodization may be set to 10˜40° C. The optimum temperature of the electrolyte is preferably 15˜30° C. When anodization is carried out using the electrolyte composed of oxalic acid, the color of the resulting oxide film may be further darkened in proportion to an increase in the electrolyte temperature. Even when the electrolyte temperature is higher than 30° C., the extent of darkening the color of the oxide film may decrease. Taking into account the maximal darkening of the oxide film, increasing the electrolyte temperature in excess of 40° C. is unnecessary.

The anodization processing time is preferably 30 min or longer. As the anodization processing time increases, the resulting oxide film may become thick and thus the radiation heat flux may increase. In particular, an anodization processing time exceeding 30 min may result in maximized radiation heat flux.

The reason why the electrolyte temperature and the processing time upon anodizaiton are limited as above is described later through the following experiments.

Also in the method, sealing is a step of sealing the oxide film formed by anodization, so that the surface color of the aluminum material is closer to black. This sealing step may include primarily immersing the anodized aluminum material in a cobalt acetate solution and secondarily immersing the primarily immersed aluminum material in an ammonium sulfide solution.

For primary immersion, the concentration of cobalt acetate (Co(CH₃COO)₂) of the cobalt acetate solution is 100˜250 g/L, and the temperature of the cobalt acetate solution is 20˜50° C., and the immersion time is preferably set to 10˜30 min.

For secondary immersion, the concentration of ammonium sulfide ((NH₄)₂S) of the ammonium sulfide solution is 10˜50 g/L, and the temperature of the ammonium sulfide solution is 20˜50° C. The immersion time is 10˜30 min.

The effects depending on the concentration of the immersion solution, the temperature and the immersion time in the primary and the secondary immersion procedure are proven through the following experiments.

Below is a description of the effects of the invention.

Experimental Example 1

Comparative examples for anodizing an aluminum material using as a conventional electrolyte a sulfuric acid aqueous solution, and examples for anodizing an aluminum material using an electrolyte composed exclusively of oxalic acid according to the present application, were performed at different electrolyte temperatures for different processing times. Then, the surfaces of the oxide films formed on the aluminum materials were compared.

In the comparative examples, sulfuric acid had a concentration of 15 wt %, and in the examples, oxalic acid had a concentration of 0.3 M. In all the comparative examples and the examples, the temperature of the electrolyte was changed to 0° C., 15° C. and 30° C., and the processing time was changed to 10 min, 20 min, 30 min and 40 min.

The results are shown in FIGS. 1A and 1B.

FIG. 1A illustrates the surfaces of oxide films depending on the temperature and time period when conventional anodization using a sulfuric acid electrolyte was performed, and FIG. 1B illustrates the surfaces of oxide films depending on the temperature and time period when anodization using an oxalic acid electrolyte according to the present invention was performed.

As illustrated in FIG. 1A, in the comparative examples using sulfuric acid as the electrolyte, the surface colors of the oxide films were gradually darkened in proportion to a decrease in the electrolyte temperature.

Further, in the comparative examples, the longer the processing time, the darker the surface colors of the oxide films.

Whereas, as illustrated in FIG. 1B, in the examples using oxalic acid as the electrolyte, the surface colors of the oxide films were gradually darkened with an increase in the electrolyte temperature.

Also in the examples, as the processing time was increased, the surface colors of the oxide films were gradually darkened as in the comparative examples. Further, when a processing time of 30 min and a processing time of 40 min were applied, there was little difference between the surface colors of the aluminum materials.

Hence, in the present application using oxalic acid as the electrolyte, it can be confirmed that the electrolyte temperature of 10˜40° C. and preferably at 15˜30° C. and also the processing time of 30 min or longer are preferable, taking into consideration heat dissipation performance.

Experimental Example 2

The relative radiation values of the oxide films formed on the aluminum materials of the comparative examples and the examples in Experimental Example 1 were measured. The results are shown in Table 1 below and FIGS. 2A and 2B.

FIG. 2A illustrates the relative radiation results of the oxide films depending on the temperature and time period when conventional anodization using a sulfuric acid electrolyte was performed, and FIG. 2B illustrates the relative radiation results of the oxide films depending on the temperature and time period when anodization using an oxalic acid electrolyte according to the present invention was performed.

TABLE 1 Examples Comp. Examples Radiation (0.3M Oxalic acid) (15 wt % sulfuric acid) (W/m²) 0° C. 15° C. 30° C. 0° C. 15° C. 30° C. 10 min 317.52 330.75 330.75 255.78 255.78 269.01 20 min 352.8 330.75 366.03 255.78 269.01 171.99 30 min 282.24 379.26 379.26 269.01 282.24 171.99 40 min 330.75 379.26 379.26 317.52 282.24 233.73

As is apparent from Table 1 and FIGS. 2A and 2B, the radiation heat flux was relatively higher in the examples than in the comparative examples when the same electrolyte temperature and the same processing time were employed.

In the examples, as the processing time was longer at high electrolyte temperature, the radiation heat flux was increased. As such, when the electrolyte temperatures were 15° C. and 30° C. and the processing times were 30 min and 40 min, the radiation heat flux values were the same within a measurement error range. Considering the profitability of the process, when anodization is performed using the electrolyte composed of 0.3 M oxalic acid, the electrolyte temperature of 15° C. and the processing time of 30 min are regarded as the most appropriate.

Experimental Example 3

To evaluate, depending on the sealing treatment, a difference in the surface colors of the aluminum materials subjected to anodization for 60 min using the electrolyte composed of 0.3 M oxalic acid at 15° C. according to the present application, the as-anodized aluminum material using the oxalic acid electrolyte before sealing treatment, the sealed aluminum material resulting from a boiling water sealing process, the sealed aluminum material resulting from a low-temperature sealing process (NiF₂) and the sealed aluminum material according to the present invention (a black sealing process) were prepared, and the surface colors thereof were observed and the radiation heat flux values were measured. The results are shown in FIG. 3.

For a boiling water sealing process, the anodized aluminum material was immersed for 30 min in deionized water at 95° C.

For a low-temperature sealing process, the anodized aluminum material was immersed for 30 min in an immersion solution comprising 3 g/L nickel fluoride (NiF₂) at 25° C.

For a black sealing process according to the present application, the anodized aluminum material was immersed for 20 min in an immersion solution comprising 200 g/L cobalt acetate (Co(CH₃COO)₂) at 45° C., and then immersed for 15 min in an immersion solution comprising 30 g/L ammonium sulfide ((NH₄)₂S) at 25° C.

FIG. 3 illustrates the surface photographs and the relative radiation results of the aluminum materials after conventional sealing treatment and the sealing treatment according to the present application. As illustrated in FIG. 3, the aluminum materials sealed by the boiling water sealing process and the low-temperature sealing process had a darker surface color than the non-sealed aluminum material, but the surface color of the aluminum material subjected to black sealing was the closest to black.

Based on the results of measurement of the radiation heat flux, the aluminum material sealed by the boiling water sealing process had the same radiation heat flux as the pre-sealing result. The aluminum material sealed by the low-temperature sealing process had improved radiation heat flux compared to the pre-sealing treatment, but the radiation heat flux of the aluminum material sealed by the black sealing process was most improved. Hence, when the surface of the aluminum material is sealed using a black sealing process, the radiation heat flux can be confirmed to be quite high, compared to the other sealing processes.

Experimental Example 4

Upon primary immersion for sealing treatment using a black sealing process, changes in the radiation heat flux depending on the concentration of cobalt acetate (Co(CH₃COO)₂) of a cobalt acetate solution were evaluated. Specifically, during a primary immersion process in the course of sealing the aluminum material subjected to anodization using the oxalic acid electrolyte, the amount of cobalt acetate (Co(CH₃COO)₂) in cobalt acetate solutions was changed. As such, the temperature of all the cobalt acetate solutions was set to 45° C., and the immersion time was set to 20 min. During the next secondary immersion process, the aluminum material was immersed for 15 min in an immersion solution comprising 30 g/L ammonium sulfide ((NH₄)₂S) at 25° C. The results are shown in FIG. 4A.

As illustrated in FIG. 4A, when cobalt acetate was used in amounts of 100, 200 and 250 g/L, the radiation heat flux was 400 W/m² or greater. Hence, the concentration of cobalt acetate (Co(CH₃COO)₂) of the cobalt acetate solution in the primary immersion process can be confirmed to be 100˜250 g/L.

Experimental Example 5

Upon secondary immersion for sealing treatment using a black sealing process, changes in the radiation heat flux depending on the concentration of ammonium sulfide ((NH₄)₂S) of an ammonium sulfide solution were evaluated. The aluminum materials subjected to anodization using the oxalic acid electrolyte underwent black sealing. Specifically, after primary immersion for 20 min in the immersion solution comprising 200 g/L cobalt acetate (Co(CH₃COO)₂) at 45° C., the aluminum materials were secondarily immersed under the conditions that the temperature of all ammonium sulfide solutions was set to 25° C. and the immersion time was 15 min while changing the amount of ammonium sulfide ((NH₄)₂S) in the ammonium sulfide solutions. The results are shown in FIG. 4B.

As illustrated in FIG. 4B, when ammonium sulfide was used in amounts of 10, 30 and 50 g/L, the radiation heat flux was 400 W/m² or greater. Thus, in the secondary immersion process, the concentration of ammonium sulfide ((NH₄)₂S) of the ammonium sulfide solution can be confirmed to be 10˜50 g/L.

Although the preferred embodiments of the present application depicted in the drawing have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the subject matter as disclosed in the accompanying claims. 

What is claimed is:
 1. A method of surface-treating an aluminum material for dissipating heat, the method comprising steps of: anodizing an aluminum material with an electrolyte comprising oxalic acid; and sealing a surface of the aluminum material by formation of cobalt sulfide (CoS) in surface pores of the aluminum material.
 2. The method of claim 1, wherein the oxalic acid of the electrolyte upon anodizing has a concentration of 0.2˜0.8 M.
 3. The method of claim 1, wherein the electrolyte upon anodizing has a temperature of 10˜40° C.
 4. The method of claim 1, wherein anodizing is performed for at least 30 min.
 5. The method of claim 1, wherein the sealing step comprises: primarily immersing the anodized aluminum material in a cobalt acetate solution; and secondarily immersing the aluminum material in an ammonium sulfide solution.
 6. The method of claim 5, wherein upon the primary immersion, cobalt acetate (Co(CH₃COO)₂) of the cobalt acetate solution has a concentration of 100˜250 g/L.
 7. The method of claim 5, wherein upon the secondary immersion, ammonium sulfide ((NH₄)₂S) of the ammonium sulfide solution has a concentration of 10˜50 g/L.
 8. A method of surface-treating an aluminum material for dissipating heat, the method comprising steps of: anodizing an aluminum material with an electrolyte comprising oxalic acid and a second acid selected from sulfuric acid, phosphoric acid or chromic acid, wherein a concentration of the second acid is between 0.1 to 1 M; and sealing a surface of the aluminum material by formation of cobalt sulfide (CoS) in surface pores of the aluminum material.
 9. The method of claim 8, wherein the oxalic acid of the electrolyte upon anodizing has a concentration of 0.2˜0.8 M.
 10. The method of claim 8, wherein the electrolyte upon anodizing has a temperature of 10˜40° C.
 11. The method of claim 8, wherein anodizing is performed for at least 30 min.
 12. The method of claim 8, wherein the sealing step comprises: primarily immersing the anodized aluminum material in a cobalt acetate solution; and secondarily immersing the aluminum material in an ammonium sulfide solution.
 13. The method of claim 12, wherein upon the primary immersion, cobalt acetate (Co(CH₃COO)₂) of the cobalt acetate solution has a concentration of 100˜250 g/L.
 14. The method of claim 12, wherein upon the secondary immersion, ammonium sulfide ((NH₄)₂S) of the ammonium sulfide solution has a concentration of 10˜50 g/L. 